When a camera is unlicensed from NetBotz Central/ StruxureWare Data Center Expert Surveillance, any existing Surveillance pictures associated with this camera is purged from the server's database.
To retain the historical Surveillance pictures for a camera without having it posted future Surveillance pictures to the server, set "Post Mode" to 'Disabled' in Modify Camera Surveillance Settings. This way, no more new Surveillance pictures are generated from the camera, and users can still review past Surveillance pictures associated with this camera.
Note that only unlicensing cameras can free up Surveillance license nodes.